Alkenyl acetates are an important monomer building block in the synthesis of plastic polymers. The main fields of use of alkenyl acetates are i.a. the preparation of polyvinyl acetate, polyvinyl alcohol and polyvinyl acetal and also co- and terpolymerization with other monomers such as for example ethylene, vinyl chloride, acrylate, maleinate, fumarate and vinyl laurate.
Alkenyl acetates are prepared for example in the gas phase from acetic acid and ethylene by reaction with oxygen, wherein the catalysts used for this synthesis preferably contain Pd as active metal, Au as promoter and an alkali metal component as co-promoter, preferably potassium in the form of the acetate. In the Pd/Au system of these catalysts, the metals Pd and Au are not present in the form of metal particles of the respective pure metal, but rather in the form of Pd/Au-alloy particles of possibly different composition, although the presence of unalloyed particles cannot be ruled out. As an alternative to Au, for example Cd or Ba can also be used as co-promoter.
Currently, alkenyl acetates are predominantly prepared by means of so-called shell catalysts in which the noble metals Pd and Au do not fully penetrate the catalyst support formed as a shaped body, but rather are contained only in an outer area (shell) of greater or lesser width of the catalyst support shaped body (cf. on this EP 565 952 A1, EP 634 214 A1, EP 634 209 A1 and EP 634 208 A1), while the areas of the catalyst support lying further inside are free of noble metals. With the help of shell catalysts, a more selective reaction control is possible in many cases than with catalysts in which the supports are impregnated into the core of the support with active component (“impregnated through”).
The shell catalysts known in the state of the art for the preparation of alkenyl acetates can be for example catalyst supports based on silicon oxide, aluminium oxide, aluminosilicate, titanium oxide or zirconium oxide (cf. on this EP 839 793 A1, WO 1998/018553 A1, WO 2000/058008 A1 and WO 2005/061107 A1). Catalyst supports based on titanium oxide or zirconium oxide are currently scarcely used, however, since these catalyst supports display no long-term resistance to acetic acid and are relatively expensive. In the case of shell catalysts coated with Zr suspension, problems with the abrasion resistance and mechanical stability arise.
The great majority of the catalysts currently used for the preparation of alkenyl acetates are shell catalysts with a Pd/Au shell on a porous amorphous aluminosilicate support formed as a sphere based on natural sheet silicates which are impregnated through with potassium acetate as co-promoter.
Such alkenyl acetate shell catalysts are usually prepared by the so-called chemical route in which the catalyst support is [steeped in] solutions of corresponding metal precursor compounds, for example by dipping the support into the solutions, or by means of the incipient wetness method (pore-filling method) in which the support is loaded with a volume of solution corresponding to its pore volume.
The Pd/Au shell of the catalyst is produced for example by first steeping the catalyst support in a first step in a Pd salt solution and then in a second step fixing the Pd component with a base, e.g. NaOH, onto the catalyst support in the form of a Pd-hydroxide compound. In a subsequent, separate third step, the catalyst support is then steeped in an Au-salt solution and then the Au component is likewise fixed by means of a base. After the fixing of the noble-metal components in the outer shell of the catalyst support, the support is washed, then dried, optionally calcined and finally reduced. The thus-produced Pd/Au shell is usually approximately 100 μm to 500 μm thick.
Usually, the catalyst support loaded with the noble metals is loaded with potassium acetate after the last fixing or reducing step wherein, rather than the loading with potassium acetate taking place only in the outer shell loaded with noble metals, the catalyst support is completely impregnated through with the co-promoter. A spherical support called “KA-160” from SÜD-Chemie AG, Munich, Germany, based on a natural sheet silicate, which has a BET surface area of approximately 160 m2/g, is predominantly used as catalyst support.
The alkenyl acetate selectivities achieved by means of the shell catalysts known in the state of the art based on Pd and Au and also KA-160 supports are approximately 90 mol-%, relative to the supplied ethylene, wherein the remaining 10 mol-% of the reaction products are essentially CO2 which is formed by total oxidation of the organic educts/products.
To increase the activity of alkenyl acetate catalysts, before the deposition of the noble metal, catalyst supports free of active metal, based on natural sheet silicates, are first surface-doped with zirconium cations. For this, for example ready-formed catalyst support shaped bodies are impregnated with a solution of a zirconium salt compound and then calcined. Although, compared with the corresponding catalysts without zirconium doping, such catalysts are characterized by increased activity with respect to alkenyl acetate production, the increased activity is not of long duration, as because of the corrosive atmosphere in alkenyl acetate synthesis the Zr is released relatively rapidly from the catalyst support and is therefore no longer available for activation of the catalyst. The zirconium applied by impregnation is not sufficiently resistant to acetic acid under process conditions even after the calcining of a modified support.